This invention relates to a hydraulic damper for use in a vehicle and particularly to a hydraulic damper of the kind including a cylinder receiving therein hydraulic liquid, a piston working in the cylinder and partitioning the interior of the cylinder into first and second chambers, a piston rod secured to the piston and extending through the first chamber to the outside of the damper, a damping force generating valve device mounted on the piston for generating damping force in the extension and contraction strokes of the damper, and a damping force adjusting mechanism for adjusting the damping force in response to, such, as the running condition of the vehicle.
The damping force generating device may be a single valve acting in both of the extension and contraction strokes or may be formed of two separate valves acting respectively in the extension and contraction strokes and preventing liquid flow in the respective reverse directions.
A hydraulic damper of the aforementioned kind is widely used in vehicles such as automobiles. The projecting end of the piston rod is usually connected to a sprung mass of the vehicle such as the chassis and the lower end of the cylinder is secured to an unsprung mass such as a wheel axle.
The damping force adjusting mechanism make it possible to adjust the damping force of the damper such that the damping force is reduced when the vehicle is running on a smooth road, thereby improving driving comfort, and that the damping force is increased when the vehicle is running on a rough road condition, thereby preventing excessive vertical movement of the chassis and preventing the bottoming phenomenon.
Typically, the damping force adjusting mechanism comprises a control rod extending rotatably through the piston rod, a by-pass passage for connecting the first and second chambers and by-passing the damping force generating valve or valves on the piston, and an adjusting valve or a shutter secured to the inner end of the control rod for adjusting the effective area of the by-pass passage in response to the rotation of the control rod. Further, a check valve is usually provided for changing the damping force between the contraction and extension strokes of the damper such that the damping force in the contraction stroke of the damper is about one half to one third (1/2-1/3) of the damping force in the extension stroke.
When the adjusting valve is adjusted to increase the passage area of the by-pass passage, the damping force decreases both in the extension and contraction strokes, whereby the desired ratio between the damping forces in the contraction and extension strokes cannot be maintained, and the decrease in the damping force in the extension stroke deteriorates the stability of the driving characteristics. It has been experienced that when the damping force of the damper is too small or the characteristic of the damper is too soft and the vehicle is running at a high speed, it is difficult to control or maintain the direction of the vehicle.
The check valve closes in the extension stroke and opens in the contraction stroke, and thereby reduces liquid flow in the by-pass passage in the extension stroke as compared with the contraction stroke.
Various proposals have been made with respect to such hydraulic dampers, and in one such proposal it is not possible to change the damping force both in the extension and contraction strokes at desired multiple steps independently.
One of the objects of the present invention is to provide a hydraulic damper wherein the damping force in the extension stroke and that in the contraction stroke can be set at a plurality of steps and independently from each other, whereby it is possible to obtain optimum damping force with respect to a particular running condition of the vehicle.
Another object of the invention is to provide a hydraulic damper having a damping force adjusting mechanism which is simple in construction and compact in size, particularly in axial length.